>None of those are reasonable or reliable indications that he/she is lying about having forgotten a password.

I agree fully, forgetting passwords happens. My AD account at work has a short, random password with a forgiving lifespan. Even though I have to enter it a few dozen times a day, following the long weekend I had for the 4th it took me 5 minutes of sitting at my desk before I remembered enough to make a few guesses at it.

If I went more than a week or so without opening my encrypted partition, the password is long, complicated and changed frequently enough that I will most likely forget it. I currently only know it by muscle memory.